MUTATIONS IN THE NPH1 LOCUS OF ARABIDOPSIS DISRUPT THE PERCEPTION OF PHOTOTROPIC STIMULI

The phototropic response is an important component of seedling establi shment in higher plants because it orients the young seedlings for max imal photosynthetic light capture. Despite their obvious importance, l ittle is known about the mechanisms underlying the perception and tran sduction of the light signals that induce phototropic curvatures. Here , we report the isolation of eight mutants of Arabidopsis that lack or have severely impaired phototropic responses. These nph (for nonphoto tropic hypocotyl) mutants comprise four genetic loci: nph1, nph2, nph3 , and nph4. Physiological and biochemical characterization of the nph1 allele series indicated that the NPH1 locus may encode the apoprotein for a dual-chromophoric or multichromophoric holoprotein photorecepto r capable of absorbing UV-A, blue, and green light and that this photo receptor regulates all the phototropic responses of Arabidopsis. It ap pears that the NPH1 protein is most likely a 120-kD plasma membrane-as sociated phosphoprotein because all of the nph1 mutations negatively a ffected the abundance of this protein. In addition, the putative NPH1 photoreceptor protein is genetically and biochemically distinct from t he HY4 protein, which most likely acts as a photoreceptor for blue lig ht-mediated hypocotyl growth inhibition. Furthermore, the NPH1 and HY4 proteins are not functionally redundant because mutations in either g ene alone affect only one physiological response but not the other, th us providing strong support for the hypothesis that more than one blue light photoreceptor is required for the normal growth and development of a seedling.